Various techniques are known for the production of nitrogen and carbon dioxide from exhaust products derived from the combustion of a hydrocarbon fuel and air; and the present invention is concerned with improvements in a number of aspects of such techniques, relating particularly to the combustion technique employed for producing the exhaust products from which the gases are derived, relating to certain aspects of the gas separation, including generation of heat employed in the separation, and still further, to a novel system providing the energy required for driving compressors ordinarily employed to compress the separated nitrogen and carbon dioxide.
In various prior art techniques, a hydrocarbon fuel and air are burned in a boiler or other device having a combustion chamber; and the products of combustion are then treated to separate the nitrogen and carbon dioxide, either or both of these gaseous components being subjected to purification stages and ultimate delivery through compressors to storage tanks or other points of use. In typical cases of this prior art type, the products of combustion originate as flue gas and not only contain substantial amounts of nitrogen and carbon dioxide, but also contain appreciable quantities of oxygen (as excess air) and quite commonly an objectionable and dangerous quantity of carbon monoxide. Because of these factors, the production of the nitrogen and carbon dioxide is undesirably diminished; and in addition, special techniques may be required in order to eliminate or separately dispose of the objectionable carbon monoxide, and also the excess oxygen.
One important object of the invention is to provide a novel combustion system which is effected in two stages in a manner providing exhaust products of combustion from the second stage or phase of the combustion, which exhaust products are characterized by capability of greater production of nitrogen and carbon dioxide per pound of fuel and also by a striking improvement in the composition of the exhaust products delivered from the second phase of the combustion. Indeed, the products of the dual phase combustion according to the present invention are comprised almost solely of nitrogen, carbon dioxide and water vapor, with a minor fraction of a percentage point of oxygen, traces of the rare gases, such as helium, argon, etc., and usually, without any measurable quantity of carbon monoxide.
In accordance with the invention, the first phase of the combustion is effected in one or more combustion type prime movers or engines, for instance, an engine of the diesel type, the exhaust products from such an engine (containing free oxygen) being delivered to a combustion unit of special construction for effecting the second stage of combustion. Additional hydrocarbon fuel is introduced into the second stage combustion unit along with the engine exhaust products, and the resultant second stage combustion products have the improved composition above referred to. Still further, this two-stage system is utilized in a novel manner in another respect in connection with the overall production of the desired gases in that the engine or engines are utilized to drive one or more compressors, thereby making use of the shaft power derived from the first stage of combustion; and the compressor or the compressors are employed to compress the separated gases. This results in a very large power economy in a plant for producing gases from fuel and air. In contrast, if electrically driven, the power consumed to drive the compressors frequently amounts to a substantial portion of the total overall cost of the production operation in various of the known prior art plants.
In accordance with one embodiment of the invention, the combustion prime mover or engine employed comprises at least one diesel engine delivering exhaust gas consisting primarily of nitrogen, carbon dioxide and free oxygen, the free oxygen comprising at least 6% by volume of the exhaust gas. Such engines are also characterized by delivery of exhaust products containing only a very minor fractional percentage of carbon monoxide. In a typical case, such a diesel engine, when operating at a normal rated continuous load, yields an exhaust gas of the following composition (by volume):
______________________________________ Excluding Including Water Vapor Water Vapor ______________________________________ O.sub.2 12.0123 11.343 CO.sub.2 6.5771 6.211 N.sub.2 81.4102 76.877 H.sub.2 O -- 5.568 99.9996 99.999 ______________________________________
In accordance with another embodiment of the invention, a combustion prime mover may be employed comprising a gas turbine engine; and this similarly delivers an exhaust gas also consisting primarily of carbon dioxide (2.9%), free oxygen (16.3%) and the balance nitrogen, without any significant trace of carbon monoxide.
According to the present invention, the first stage combustion products, such as those above referred to, are then subjected to a second stage of combustion in equipment of special type, as fully explained hereinafter; and additional fuel is added, thereby combining with the free oxygen in the exhaust products of the first stage.
In explanation of another important aspect of the invention, it is first pointed out that in the known technique for separating nitrogen and carbon dioxide from products of combustion, the gases being used are passed through what is known as an absorber in which the carbon dioxide is selectively absorbed, for instance, by an aqueous solution of monoethanolamine, the nitrogen passing through and out of the absorber. The monoethanolamine, rich in carbon dioxide, is delivered from the absorber into a stripper in which the solution of monoethanolamine and carbon dioxide is subjected to heat, thereby driving off the carbon dioxide, the monoethanolamine being then returned in a closed circuit for reuse in the absorber. In accordance with the present invention, the heat generated in the second phase of the dual phase combustion is utilized to heat an indirect heat transfer medium, such as water; and this medium either as hot water or as steam is employed in the stripper for the purpose of separating the carbon dioxide from the "rich" monoethanolamine solution. As is known, other absorbers may be used, for instance, other amines or potassium hydroxide. Preferably, the second phase of combustion is carried out in a unit providing for the generation of superheated water or steam; and in the stripper, in consequence of the stripping action, condensate or liquid is formed and is then returned either indirectly to the heat exchanger which is incorporated in the secondary combustion unit, or in the manner referred to just below.
When a diesel engine is employed for the first stage of combustion, the invention still further contemplates utilization of the cooling jackets of the diesel engine or engines for contributing heat to the medium for introduction into the stripper; and it is preferred to connect these cooling jackets into a return circuit flow path with the heat exchanger of the secondary combustion unit and the stripper, so that the cooled liquid or condensate is delivered from the stripper into the cooling jacket of the engine or engines constituting the first phase of the combustion system, the hot water from the engine jacket or jackets then being delivered into the heat exchanger of the secondary combustion unit, from which steam or superheated water is again delivered to the stripper. When using a diesel engine, this provides for an additional efficiency in that the heat of the engines employed for the first phase of combustion is utilized as a preheater to the heat exchanger in the secondary combustion unit, thereby effecting still another economy in the overall plant operation.